The present invention relates to improved methods for forming rare earth and alumina (Al2O3) doped glass preforms from which optical fibers can be made. More particularly, this invention discloses methods for producing high alumina-doped, yet inclusion-free, optical waveguide preforms which can be drawn into optical waveguide components, such as couplers and amplifiers.
Rare earth-doped optical fibers, such as erbium-doped glass, are suitable for many uses, particularly as optical amplifiers. These amplifiers make it possible to amplify an optical signal without first converting it into an electronic signal.
Erbium-doped fibers (xe2x80x9cEDFxe2x80x9d) are typically used in wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d) systems. WDM are high data rate systems that allow simultaneous transmission of several signals in an optical waveguide at differing wavelengths. These systems usually include a source that can send signals at multiple wavelengths or input channels, a multiplexer, an optical fiber cable, a demultiplexer, and multiple output sources or output channels.
An EDF allows the amplification of an optical signal having a wavelength range of about 1530 to about 1610 nanometers (xe2x80x9cnmxe2x80x9d). The erbium-doped fiber acts as an amplifier when a continuous source of pump light, at a wavelength of either 980 or 1480 nm, is propagated through its length. When the optical signal is also sent through the erbium-doped fiber, the erbium ions, excited by the pump light, amplify the signal through the stimulated emission of photons from the excited state. Other rare earth dopants, such as praseodymium and neodymium, are possible candidates to amplify optical signals around the 1300 nm wavelength range.
An important parameter for an EDF is bandwidth. This allows the EDF to handle more channels, or accommodate more data, in a WDM system and similar applications.
An optical fiber preform is generally comprised of a central core and an outer cladding layer. The core has a higher refractive index than the cladding layer. When the preform is drawn into an optical fiber, the difference in refractive indices between the two layers allows the propagation of the optical signal within the core. A typical optical fiber glass core composition is comprised mainly of high purity SiO2 glass with lesser amounts of GeO2 and/or other dopants, depending upon the desired optical properties. Fibers whose cores are doped with GeO2 exhibit low loss characteristics which extend to wavelengths around 1600 nm.
Alumina is well known in the art as a co-dopant in optical fiber preform glass compositions which also contain erbium or other rare earth dopants. Alumina, when used in these compositions, enhances the processing of the fiber preform by increasing the solubility of erbium or other rare earth dopants in the resultant fiber. Optical fibers drawn from alumina-containing preforms exhibit enhanced optical properties, such as a lower ripple value and an expanded bandwidth. Higher concentrations of alumina within an EFA improves its performance by flattening the Er gain spectrum over a given bandwidth.
Despite its many advantages, higher concentrations of alumina can present difficulties during the fabrication of the glass preform and optical fiber. One of the main difficulties encountered in fabricating alumina-containing glass preforms is the formation of inclusions, seeds (gas bubbles) or crystals. The presence of inclusions within the preform can render the subsequent fiber drawing process difficult or even impossible. Inclusions can decrease the length of fiber drawn from the preform and, at worst, prohibit fiber drawing entirely. FIG. 1(a) is a photograph taken at a magnification of approximately 10 times actual size that illustrates the seeds present in a Type 3 (see supra Table I in Detailed Description) glass preform made from methods known in the art. Preforms containing many inclusions are usually scrapped, thereby increasing manufacturing costs. If the preform can be drawn into a fiber, inclusions can be a major source of attenuation loss due to scattering and decreased strength in the resulting fiber. For the manufacture of high quality preforms, particularly core preforms known as rods or canes, the reduction, and preferably, the elimination of inclusions is of critical importance.
There are many proposed mechanisms of bubble formation in alumina-containing glass preforms. Glass preforms are usually formed by a chemical vapor deposition process. Oftentimes, when the dopant system includes solids rather than liquids, such as AlCl3, solid dopant particles can be carried to the reaction zone by carrier gases during the soot lay-down step. Since the dwell time of solid particles in the heat source or flame is minimal, the solid particles cannot be completely reacted, or oxidized. These unreacted solid particles may attach to the glass particulate, or soot, and become incorporated into the soot blank. The particles eventually react and decompose in subsequent processing steps involving elevated temperatures. For example, during the consolidation step, solid particles of the aluminum-containing precursor such as AlCl3 can react with oxygen at temperatures which range from about 1400 C to about 1500 C to form alumina Al2O3. The decomposition of these solid particles causes gas bubbles of Cl2 to form in the resultant preform or preform core.
In preforms that contain both GeO2 and Al2O3 as dopants, gas bubble formation can result from, or be aggravated by, GeO2 thermal decomposition. During the drawing or re-drawing step, the GeO2 particles within the soot blank are driven to convert into their gaseous phase and form gas bubbles within the body of the preform or preform core. These further contribute to seed formation.
Subsequent anneal processing may be employed to reduce or eliminate the gas bubbles that are formed. For alumina doped preforms, additional heat processing may cause the unwanted formation of Al2O3 crystals, namely cristobalite or mullite, within the glass. Lower temperatures and Al2O3 concentrations within the composition tend to form cristobalite, whereas higher temperatures and higher Al2O3 concentrations tend to form mullite and cristobalite. The likelihood of crystallite formation increases as the concentration of alumina dopant increases. Crystallites within the preform, like gas bubbles or seeds, can inhibit or prevent the preform from being drawn into fiber. Further, crystallites can become a scattering site and degrade the resulting optical properties of the fiber.
The present invention improves the performance of an erbium doped fiber by increasing the concentration of alumina dopant within the optical fiber waveguide preform core. The present invention also provides methods for producing a rare earth and high concentration alumina doped optical fiber preforms which are substantially inclusion free. The methods of the present invention are particularly useful for making optical waveguide fibers drawn from a glass comprising Al2O3, GeO2, and SiO2, which can be used as host glass containing dopants such as erbium or other rare earths for making optical amplifiers.
The method of the present invention allows for high alumina doping yet substantially eliminates inclusions in the glass preform or cane by modifying certain process parameters during the soot lay-down step. During this step, a glass particulate, or soot blank, is created by reacting glass constituents in vapor form with oxygen via a heat source, such as a flame burner, in the reaction zone. The soot particles are deposited onto the surface of a rotating, removable mandrel. The burner or torch traverses across the length of the rotating mandrel, thus allowing a uniform deposit of soot along its surface. Unlike many other glass precursors or dopants, the aluminum-containing glass precursor, such as AlCl3, is in solid, rather than liquid, form. The present method increases the temperature range at which the aluminum-containing solid is sublimated from about 130 C to about 170 C, or more preferably, from about 150 C to about 170 C. The increased sublimation temperatures result in an increase in the concentration of alumina in the resultant soot blank. The aluminum-containing vapor is transported from the sublimator to the reaction zone via a carrier gas, such as an helium, argon, or other inert gases. In addition, a further embodiment of the present invention reduces the flow rate of the carrier gas by more than half, from its typical value which ranges from about 1.4 to about 1.8 standard liter per minute (xe2x80x9csplmxe2x80x9d) to about 0.5 to about 0.7 splm. In yet another embodiment of the present invention, the traverse speed of the heat source or burner across the soot blank is increased from about 2 cm/sec to a range of about 2.5 cm/sec to about 3.5 cm/sec, more preferably 3.0 cm/sec, to prevent overheating of the soot blank.
The present invention also eliminates inclusions in the preform by modifying the consolidating or sintering step. As mentioned earlier, the presence of alumina and germania as dopants within the preform core can cause competing reactions which foster inclusions in the resultant preform or cane. Aluminum-containing solid particles are transported into the soot blank rather than oxidized in the reaction zone. These particles eventually form Al2O3 by subsequent heating. However, subsequent heating, at higher temperatures to oxidize the aluminum-containing particles, causes the GeO2 in the soot blank to thermally decompose or convert to its gaseous phase. In order to prevent GeO2 thermal decomposition, the temperature of the consolidating step is reduced to a range from about 1400xc2x0 C. to 1500xc2x0 C., more preferably from about 1400xc2x0 C. to about 1450xc2x0 C., compared with a typical sintering temperature of about 1495xc2x0 C. In addition, the centerline of the soot blank is cooled by a high helium or other inert gas flow at about 1 to about 5 slpm, preferably between about 1.5 to about 3.0 slpm. This flow rate is higher than the normal flow rate. The combination of lower consolidation temperatures and a higher helium or inert gas flow prevents GeO2 thermal decomposition from occurring during the consolidating step.
Lastly, the drawing step of the consolidated glass blank is modified by conducting two or more drawing steps. This process, known as the double re-drawing process, is disclosed in pending patent application Ser. No. 09/318,316 and is incorporated herein by reference in its entirety. The additional drawing step or steps eliminates the need for subsequent processing steps to remove inclusions within the glass core or preform, thereby reducing manufacturing time and cost. The consolidated glass blank is initially drawn at a first temperature, or a glass temperature ranging from about 1600 C to about 1900 C. A vacuum of about 100 torr or higher may be drawn along the centerline of the glass blank to circulate the Ar gas in the center aperture and drive off any residual water within the blank. The first draw step yields a glass preform, or cane, precursor which may contain a elongated seed or aperture along its centerline. The diameter of the consolidated glass blank is about 30 to 50 mm. After the initial drawing, the diameter of the resulting glass preform or cane precursor is about 20 to 35 mm.
The redrawing, or second drawing, step is performed at a second temperature, or a glass temperature ranging from about 1600 C to about 1800 C and produces a glass perform precursor or cane precursor. The second drawing step is preferably conducted under vacuum conditions applied along the centerline of glass preform or cane precursor. In a preferred embodiment of the present invention, the first drawing step is conducted at a higher temperature but a lower vacuum than the second drawing step. The higher vacuum in the second drawing step aids in removing the aperture along the centerline of the glass preform or cane precursor by closing one end of the aperture and evacuating the aperture. Thus, the resulting canes or preforms are substantially inclusion-free yet contain a higher concentration of alumina.
Several important advantages are achieved by making optical waveguide preforms or waveguide core preforms (also referred to as canes or rods) using the methods of the invention. For example, one advantage of the present invention is providing a preform or cane that is substantially free of inclusions, such as air bubbles and crystallites. FIG. 1(b) is a photograph, at a magnification of approximately 10 times actual size, that illustrates the substantial absence of seeds present in a Type 3 (see supra Table I in Detailed Description) glass preform made from the present invention. A waveguide core preform free of such inclusions will require fewer processing steps to produce waveguide fiber, and waveguide fiber drawn therefrom will contain less scattering sites and thus have a lower attenuation. In addition, the methods are reproducible and enhance the utilization of the preform for subsequent fiber drawing. For example, a preform manufactured from one process of the invention can be used to draw 60 km to 360 km lengths of fiber from one preform. Preforms made from analogous methods of the art may yield far lower fiber lengths or, in worst case, be unusable for drawing fiber. Because greater lengths can be drawn from one preform, fiber costs are reduced. Fiber costs are also reduced due to the reduction of waste or scrap.
One embodiment of the present invention describes a method to produce Er2O3xe2x80x94Al2O3xe2x80x94GeO2xe2x80x94SiO2 fiber preforms with virtually no inclusions. The method disclosed is reproducible and can be scaled upward to meet production demands. The present invention can also be applied to other rare earth doped fiber compositions and preform fabrication processes.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention. In the drawings, like reference characters denote similar elements throughout several views. It is to be understood that various elements of the drawings are not intended to be drawn to scale.